Honestly, the ballads on a given DT album could be a highlight of the record, just depends on the album and song itself. Not all of these I would staright-away classify as "ballads", but are at least typically the least metal, softest songs on the album.
I&W: Wait For Sleep. Pretty obvious choice but it really is fantastic and beautiful and gorgeous. Ballad or not, is one of my favorite DT songs in general.
Awake: Space-Dye Vest, naturally. Another obvious choice, but such a stand-out "DT" song.
FII: Hollow Years is really the only one here I enjoy quite a bit. It's just so catchy and emotionally gripping at points, I can't ignore it.
SFAM: Pretty much all of the ballads on here are great, but if I had to go with one, I'd go with Through Her Eyes. Such a sad tear-jerker, and a perfect closing out of the first act before the story picks up again.
6DOIT: This one's a bit tricky. Does Disappear count? Cause either way it's a great song. An emotionally devastating dirge of a track. If we were to separate the title track into its parts, then Goodnight Kiss is a good contender as well.
ToT: Vacant. Similar to how I feel about Disappear, I love how it serves as a dark, despondent prelude to Stream of Consciousness.
8V: I Walk Beside You. I can already see the eyebrows being raised.
Probably not the softest cut on 8V, but it is my favorite of that category. Kinda like with Hollow Years, I adore some of the catchy hooks on here as well as some of the more contemporary instrumental choices.
SC: Closest thing that fits in this category would be Repentance. Which I like just fine if it were cut down a few minutes. I'd love to include Ministry of Lost Souls were it not for that gaudy instrumental section.
BC&SL: Wither is decent I suppose, in spite of it just being "Writer's Block: The Song".
ADTOE: The 3 ballads here I like more or less equally, but I'd have to give it to This Is the Life by a smidge. I don't feel real strongly about any of them, but they're just fine.
DT12: Again, don't feel too strongly about the ballad on here. I hardly go out of my way to listen to it on it's own, not in context with the rest of the album. Could we count the hidden track at the end of Illumination Theory?
TA: To be honest, I couldn't be bothered to seek out a favorite ballad on here with the over abundance of them on this album, none of which left any impression on me when they start to blend together.